1. Field of the Invention
The invention relates to an optical character generator for an electrographic printer having a plurality of monolithic circuits which are arranged on a carrier, each monolithic circuit comprising LEDs which are integrated therein and can be driven individually, and a drive circuit which is assigned to the character generator.
2. Description of the Related Art
Such an optical character generator is disclosed in European Patent Document EP-A2-0 180 479.
LED array character generators such as those which are also described, for example, in European Patent Document EP-B1-0 403 476 are used in non-mechanical printers for point-by-point discharging of electrostatic charged photoconductors. The latent electrostatic image produced in this way is inked in with the aid of a developer station and is transfer-printed onto the recording medium in a transfer-printing station. LED array character generators, which are supplied with drive signals via a drive circuit and a BUS circuit, are used to produce the charge image. Both the drive circuit and the BUS circuit are normally designed as separate integrated circuits. The ICs are constructed on the basis of monocrystalline silicon and contain a large number of transistors for analog/digital conversion (a switch function) and for amplification of currents (a driver function), which are required for the supply of the LED arrays.
As a rule, the LED arrays comprise 8, 16, 32, 64 or 128 light-emitting diodes which are arranged in series in a row (also termed the x-direction) and are integrated in one or more rows on individual monolithic circuits. A plurality of monolithic circuits which are joined to one another form the character generator. The structuring of the diodes is carried out on the basis of gallium arsenide. Each diode contains a connection pad via whose connection a current is passed and the diode is thus energized to illuminate. The light energy for exposure of the photoconductor is controlled via the magnitude of the current and via the time over which the diode is energized. Each individual LED is to this end connected to in each case one amplifier output of an IC. For this purpose, each IC is positioned very close to the LED arrays, so that a connection pad of an amplifier output of the IC is opposite each connection pad of an LED array. The pads are now connected to one another with the aid of automatic bonding machines. Since each character generator requires up to 11,000 diodes, and thus a correspondingly high number of bonded joints, depending on the length and the diode separation, separation into an LED chip and an IC and the connection technology which would have to be used for this purpose are not only very time-consuming but, furthermore, are also highly susceptible to defects. For example, a 300 dpi character generator has a distance of 85 .mu.m between centers of the light-emitting diodes. The width of a connection pad can therefore be no greater than 60 .mu.m if a safety margin of 25 .mu.m is intended to remain between each pad. The positioning accuracy of each automatic bonding machine must therefore be so great over the length of the entire character generator of, for example, 300,000 .mu.m to 500,000 .mu.m that the first connection pad at the start of the bonding process and the last connection pad after 3000 to 6000 stepping movements as well lie in a reliable bonding area of approximately .+-.25 .mu.m.
Various options are used in order to reduce the difficulties which occur during bonding. The connection pads are arranged offset in the y-direction so that they can be designed to be broader and the tolerance band for the bonding can thus be enlarged. However, this necessitates a greater area requirement in the chip surface and thus increases the price of the chips.
Another option, which is described in German Patent Document DE-C2-37 04 984, comprises the LED arrays being driven from both sides with ICs. Although this results in the opportunity to double the connection pad width, it increases the area requirement for the LED array, however, and, in addition, requires twice the number of ICs.
A further disadvantage of such LED character generators is that testing of the combination of LED arrays and ICs is not possible until the complete character generator has been assembled and the bonding process has been carried out. Defects which occur cannot be confirmed until the character generator has been completely assembled, for which reason repair by replacement of the ICs is difficult and expensive.
The dissipation of heat during the printing operation and, above all, the associated dissipation of the heat produced in the LED chips (which are monolithic circuits) to the carrier are problematic in all LED character generators. As a result of the small area of binding in the LED array, only a shrinkage-free solder joint is in general possible, as a result of which it is possible to dissipate heat in the z-direction to the cooling ribs which are possibly fitted there. A more favorable bonded joint, which is less problematic, by means of, for example, silver conductive adhesive, requires a large-area contact surface.
LED arrays having adequate radiation performance can at the moment be structured only on the basis of gallium arsenide substrates. The known silicon technology, which enables a high integration density of logic and analog structures, is once again generally used for the production of the drive circuits and of the BUS circuits. For this reason, until now, the LED chips and the ICs of the drive circuits and of the BUS circuits have had to be arranged in the described manner separately on the carrier of the character generator and have had to be linked to one another by means of a bonding technique.
European Patent Document EP-A1-0 199 852 discloses an optical character generator for an electrographic printer which comprises a monolithically integrated photodiode drive combination in the case of which both the associated LEDs and the drive circuit for the LEDs are integrated on a gallium arsenide substrate.
If optical character generators are composed of monolithic circuits having individual LED arrays, the light spot sequence with a predetermined light spot separation at the boundaries of the monolithic circuits must not be disturbed. This is particularly problematic in the case of a multi-row arrangement of the LEDs. In order to ensure this light spot sequence, European Patent Document EP-B1-0 403 476, which has already been discussed, discloses the individual monolithic circuits being separated by means of an oblique cut, and the monolithic circuits being joined to one another with a butt joint, taking account of the light spot sequence.
This necessitates a high adjustment cost during assembly, and furthermore causes material losses when the monolithic circuits are cut out of the wafer, and reduces the integration density of the LEDs.